Interpretive Summary: Cattle fever ticks (CFT), Rhipicephalus microplus and R. annulatus, are obligate ectoparasites and vectors of the infectious agents that cause bovine babesiosis or “Cattle Fever”, and anaplasmosis in cattle. The CFT and the disease they transmit caused tremendous economic damage to cattle production in the United States before the initiation of the Cattle Fever Tick Eradication Program (CFTEP) by the U.S. Department of Agriculture in 1906. After more than three decades of intense efforts, CFT were declared eradicated from the U.S. in 1943. A permanent quarantine zone on the border between Mexico and the U.S. in South Texas has been in place to prevent CFT incursion from Mexico, where CFT and bovine babesiosis are endemic. The CFTEP keeps the U.S. free of CFT and bovine babesiosis; however, the CFTEP is facing serious challenges associated with global changes. Chemical acaricides are widely used to control CFT in Mexico. Tick populations have developed resistance to almost every chemical acaricide class that is commercially available in Mexico. Acaricide resistance poses an immediate threat to the continued success of the CFTEP. In order to develop efficient strategies to mitigate acaricide resistance problems that threaten the continued success of the CFTEP, it is critical to monitor acaricide resistance situation and to understand the mechanisms of resistance in CFT from Mexico. USDA-ARS researchers and Mexican scientists developed collaborative research to understand acaricide resistance problems. The science-based knowledge generated at USDA ARS research facilities through this collaboration will help develop practical resistance management strategies to sustain the success of the CFTEP.

Technical Abstract:
Ticks and the diseases they transmit cause great economic losses to livestock in many tropical and sub-tropical countries, including Mexico. Chemical acaricides have been widely used to control the cattle tick, Rhipicephalus (Boophilus) microplus, in Mexico. Resistance to organophosphate (OP), synthetic pyrethroids (SP), amitraz, and fipronil has been reported in Mexico in the past decades. Due to problems of resistance to these acaricides, macrocyclic lactones, especially ivermectin, gained popularity among ranchers and are the most sold acaricides in the Mexican veterinary market in recent years. Ivermectin-resistant populations of R. microplus have been reported recently in the states of Veracruz and Yucatan. Although resistance levels were generally low in most cases, some field populations of R. microplus exhibited high levels of ivermectin resistance. One of the populations tested showed a resistance index (RI) of 10.2 and 79.6 at LC50 and LC99, respectively. Many field populations of R. microplus are also resistant to multiple classes of acaricides, including OP (chlorpyrifos and diazinon), SP (flumethrin, deltamethrin, and cypermethrin), amitraz, and ivermectin. Resistance to ivermectin and other macrocyclic lactone acaricides found in the Mexican cattle tick populations presents a new challenge for tick control in Mexico and complicates efforts by the USDA’s Cattle Fever Tick Eradication Program.